Tunable luminescence and energy transfer of a Eu²⁺/Mn²⁺ co-doped Sr₃NaY(PO₄)₃F phosphor for white LEDs

Abstract

A series of single-phase and color-tunable Sr₃NaY(PO₄)₃F:Eu²⁺, Mn²⁺ phosphors have been successfully prepared via the solid-state reaction. The crystal structure and phase of the prepared phosphors were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The excitation and emission spectra, and fluorescence decays were measured and discussed systematically. The results revealed that Eu²⁺ can efficiently transfer excitation energy to Mn²⁺ via its 4f states and therefore sensitizes Mn²⁺ emission under near-ultraviolet (NUV) excitation. In addition, the energy transfer mechanism was demonstrated to be a resonant type via a dipole–dipole process. The corresponding Commission Internationale deL'Eclairage (CIE) chromaticity coordinates showed that the emission colors of the phosphors can be tuned from blue (0.133, 0.148) to white (0.327, 0.351) by adjusting the ratio of Eu²⁺ and Mn²⁺. Meanwhile, the good thermal stability of Sr₃NaY(PO₄)₃F:0.03Eu²⁺, 0.12Mn²⁺ sample showed that the emission intensity at 150 °C was reduced from 100% to 72.3% measured at 25 °C. The results confirmed that the as-synthesized Sr₃NaY(PO₄)₃F:Eu²⁺, Mn²⁺ phosphors have strong potential to be used as single-phase and NUV excitable white-emitting phosphors.

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1. Introduction

White light-emitting diodes (WLEDs) which are the typical form of solid state lighting have revolutionized traditional illumination in the past few years.^1,2 Compared to the conventional incandescent or fluorescent lamps, WLEDs possess long lifetimes, high brightness, are highly efficient, environment friendly and therefore have attracted more interest in the field of solid state lighting.^3,4 The most currently used WLED is a combination of an InGaN blue LED chip with a Ce-doped yttrium aluminium garnet (YAG:Ce) phosphor.⁵ However, this method suffers from the problems of a low color rendering index (R_a < 80) and high correlated color temperature (T_c > 4500 K) because of the insufficient red light contribution.^6,7 Another approach have been used to obtain white light is the combination of ultraviolet (UV) or near-ultraviolet (NUV) LED chips having tricolor (red, green and blue) phosphors. Unfortunately, the phosphor mixture produces an inevitable problem of fluorescence re-absorption between different components and a non-uniformity of the luminescence properties, resulting in a reduced luminous efficiency and time dependent shift of the color point.^8,9 In this case, the method to carry out color-tunable white light emission in a single phase with high efficiency and excellent color rendering index is highly desirable.

As it is well known that selecting suitable activators and an appropriate host matrix is crucial in the generation of white light, therefore the Eu²⁺ and Mn²⁺ being two important activators have been widely investigated for LED applications.¹⁰ The Eu²⁺ ions being a perfect sensitizer, always show broad absorption and high efficiency emission because the 4f⁷–4f⁶5d¹ transition of Eu²⁺ ions is an electric dipole allowed transition.¹¹ Therefore, Eu²⁺ is more suitable for pumping by UV LEDs. The Mn²⁺ doped luminescent materials have been known to have a wide range of emissions from 500 to 700 nm depending on the crystal field of the host material.^12,13 The emission efficiency of Mn²⁺ singly doped phosphors is low under UV excitation, because of the hindrance of ⁴T₁–⁶A₁ transition.¹⁴ As a promising sensitizer for Mn²⁺ ions, Eu²⁺ ions have been extensively applied in many Mn²⁺-doped hosts, such as BaSi₃O₄N₂,¹⁵ Ba_1.55Ca_0.45SiO₄,¹⁶ and Ca₉La(PO₄)₇ (ref. 17) to improve the emission intensity of Mn²⁺. Moreover, white light can be realized by an effective resonance-type energy transfer in this single-phase Eu²⁺/Mn²⁺ co-doping system.

Fluoro-apatite compounds with the general formula M₃XRe(PO₄)₃L (M = Ca, Sr, Ba; X = Li, Na, K; Re = La, Gd, Y; L = F, Cl, OH) are a kind of important luminescence host, which can produce plenty of crystal field environments imposed on emission centers.^18,19 Recently, a variety of phosphates with the formula M₃NaRe(PO₄)₃F have been reported in the literature, such as Sr₃NaGd(PO₄)₃F, Ba₃NaLa(PO₄)₃F, and Sr₃NaLa(PO₄)₃F. These are being good candidates to be used as host structures due to several merits, such as high chemical stability, good thermal quenching, and intense luminescence for WLEDs application when activated by Eu²⁺ and Mn²⁺.^20–22 By partially replacing the Gd or La ions with the Y ions, a novel phase of Sr₃NaY(PO₄)₃F (SYNPF) can be formed, which is isostructural with the fluoro-apatite Sr₃NaGd(PO₄)₃F and Sr₃NaLa(PO₄)₃F.²³ To the best of our knowledge, the luminescence properties of SYNPF:Eu²⁺, Mn²⁺ phosphor have not been reported yet. In this study, we have synthesized single-phase white-light-emitting phosphors consisting of SYNPF co-doped with Eu²⁺ and Mn²⁺. The crystal structure, photoluminescence excitation and emission spectra, energy transfer efficiency and thermally dependent luminescence quenching properties were investigated in detail. The results indicate that the SYNPF:Eu²⁺, Mn²⁺ phosphor have potential applications as a single-component white-emitting phosphor for NUV-excited WLEDs.

2. Experimental section

2.1 Materials and syntheses

All of the powder samples were synthesized by a traditional solid-state reaction method. The constituent oxides or carbonates SrCO₃ (A.R. (Analytical Reagent)), SrF₂ (A.R.), Na₂CO₃ (A.R.), (NH₄)₂HPO₄ (A.R.), Y₂O₃ (99.99%), Eu₂O₃ (99.99%), and MnCO₃ (A.R.) were employed as the raw materials. These samples were weighted and mixed homogeneously using an agate mortar for 30 min. After mixing and grinding, the mixtures were taken in an alumina crucible and then sintered at 1100 °C for 4 h in a corundum crucible imbedded in active carbon. Finally, they were furnace-cooled to room temperature, and ground again into powder for the following analysis.

2.2 Characterization

The phase structure of the as-prepared phosphor was recorded by an X-ray powder diffraction spectroscopy (XRD, Bruker D2, Karlsruhe, Germany) with Cu Kα radiation (λ = 1.5406 Å) operated at 30 kV tube voltage and 10 mA tube current. The morphology of the samples was determined by using a field-emission scanning electron microscope equipped with an energy-dispersive spectrometer (FE-SEM, S-4800, Hitachi, Japan) and a transmission electron microscope (TEM, JEM-2100F, JEOL). The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the samples were measured on a fluorescence spectrophotometer (F-7000, Hitachi, Japan) with a photomultiplier tube operating at 400 V, and 150 W Xe lamp used as an excitation source. Diffuse reflection spectra were recorded using a UV-Vis-NIR spectrophotometer (UV-3700, SHIMADZU) and white BaSO₄ powder was used as a reference standard. The luminescence decay curve was obtained using a spectrofluorometer (HORIBA, JOBIN YVON FL3-21), and the 370 nm pulse laser radiation (nano-LED) was used as the excitation source. The temperature-dependence luminescence properties were measured on the same spectrophotometer, which was combined with a self-made heating attachment and a computer-controlled electric furnace (Tianjin Orient KOJI Co. Ltd, TAP-02).

3. Results and discussion

3.1 Crystal structures and phase analysis

The XRD powder was performed to verify the phase purity of the samples. Fig. 1 illustrates that the XRD patterns of SYNPF host, SYNPF:0.03Eu²⁺, SYNPF:0.15Mn²⁺ and SYNPF:0.03Eu²⁺, 0.15Mn²⁺ samples. All the diffraction peaks of the samples were well indexed to the SYNPF (JCPDS 50-1595) phase, indicating that the obtained samples were single phase and the doped ions were completely dissolved in the SYNPF host without inducing significant changes of the crystal structure.

Fig. 1 The XRD patterns of SYNPF host, SYNPF:0.03Eu²⁺, SYNPF:0.15Mn²⁺, SYNPF:0.03Eu²⁺, 0.15Mn²⁺ samples and the standard pattern of JCPDS card file of 50-1595.

To further study the structure of the obtained samples, especially the coordination environment of the Sr²⁺ ions, the Rietveld structural refinements for SYNPF were performed based on the general structure analysis system (Fullprof) program. Fig. 2 plots the experimental, calculated and difference XRD profiles along with Bragg positions for the Rietveld refinement of SYNPF at room temperature. The Rietveld refinement results and cell parameters for this phosphor are illustrated in Table 1. The phosphor was found to crystallize in the hexagonal crystal system with the space group of P3̄ (no. 147) and the cell parameters were determined to be a = b = 9.565434 Å, c = 7.103435 Å and V = 564.795 ų. The refinement finally converged to R_p = 4.64%, R_wp = 6.24%, and χ² = 2.38, indicating that all the observed peaks satisfy the reflection conditions and our prepared phosphor is of single phase. Furthermore, all atom positions, fraction factors, occupation probability and thermal vibration parameters of SYNPF host are also given in Table S1 (ESI†), and the results show that the designed chemical formula are reasonable. In addition, Fig. 3 shows the unit cell structure of SYNPF:0.03Eu²⁺ sample together with the coordination environments of the Sr²⁺ sites. In the host, the cations in the host are connected by the [PO₄]³⁻ tetrahedral formed by the P and O atoms and there is only one Sr site with six oxygen atoms and one fluorine atom coordinated. Given the identical electric charges of Sr²⁺ and Eu²⁺ and their similar radii (r_Sr²⁺ = 0.118 nm at CN = 6, r_Eu²⁺ = 0.117 nm at CN = 6), there will be one kind of Eu²⁺ emitting center generated by the substitution of Sr²⁺ ions in the SYNPF:0.03Eu²⁺ phosphor.²⁴

Fig. 2 Rietveld refinement of the XRD profile of SYNPF.

Table 1 Rietveld refinement results and crystal data for the SYNPF host

Empirical formula	SYNPF
2θ range (°)	20–90°
Crystal system	Hexagonal crystal system
Space group	P3̄ (no. 147)
a/Å	9.565434 Å
c/Å	7.103435 Å
V/Å^3	564.795 Å^3
Z	2
Rp (%)	4.64%
Rwp (%)	6.24%
χ^2	2.38

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Fig. 3 Structural view of a unit cell of SYNPF:0.03Eu³⁺ and coordination environments for Sr sites.

In order to further investigating the morphology and structure of the as-prepared samples, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) spectrum were performed on the SYNPF:0.03Eu²⁺, 0.12Mn²⁺ phosphor. From Fig. 4(a), one knows that the samples were composed of aggregated and irregular particles with sizes ranging from 0.1 to 1 μm. Furthermore, the TEM image further proved that the obtained samples were made of micro-particles (see Fig. 4(b)). According to the HR-TEM image (Fig. 4(c)), the lattice spacing was estimated to be about 3.61 Å corresponding to the (002) plane of hexagonal SYNPF:0.03Eu²⁺, 0.12Mn²⁺. These results show that well-crystallized SYNPF:0.03Eu²⁺, 0.12Mn²⁺ powders have been obtained. Meanwhile, the composition of the sample has been analysed by EDX. As shown in Fig. 4(d), we can see that the sample is composed of Sr, Na, Y, P, O, F, Eu and Mn atoms. From the result demonstrated in Table S2 (ESI†), the atomic ratio in the sample is Sr : Na : Y : P : F = 2.91 : 1 : 1.09 : 3.14 : 1.05, which is similar to their ratio in the formula.

Fig. 4 (a) SEM image, (b) TEM image, (c) HRTEM image and (d) EDX spectrum of SYNPF:0.03Eu²⁺, 0.12Mn²⁺.

3.2 Luminescent properties of Eu²⁺, Mn²⁺ and Eu²⁺/Mn²⁺ co-doped in SYNPF

Fig. 5 shows the powder diffuse reflectance spectrum (DRS) of the host, Eu²⁺ singly doped, and Eu²⁺/Mn²⁺ co-doped samples, respectively. The SYNPF host shows a plateau of high reflection in the wavelength range of 310–800 nm. While doping 3 mol% Eu²⁺ in the host, a strong and wide absorption band in the NUV range was observed, which mainly resulted from the transition of Eu²⁺ that originates from the 4f⁷ ground state to the 4f⁶5d¹ excited state. For SYNPF:0.03Eu²⁺, 0.12Mn²⁺, a similar spectrum is observed. Mn²⁺ ions may also contribute to the increased absorption intensity in the wavelength range from 200 to 450 nm by means of the metal–ligand charge transfer band of Mn²⁺–O²⁻.²⁵ The DRS of SYNPF:0.03Eu²⁺, 0.12Mn²⁺ shows slight absorption at about 420–520 nm which is ascribed to the transition from the ⁶S ground state to the ⁴G excited state in Mn²⁺. As a result, the excitation wavelength presents an overall red shift to 440 nm, which matches well with NUV chips for applications in white-light NUV LEDs.

Fig. 5 Diffuse reflectance spectra of SYNPF host, SYNPF:0.03Eu²⁺ and SYNPF:0.03Eu²⁺, 0.12Mn²⁺ phosphors; the inset shows the extrapolation of the band gap energy for the host.

The band gap of the SYNPF host can be estimated according to eqn (1):²⁶

F(R_∞hv)ⁿ = A(hv − E_g) (1)

where hv is the incident photon energy, A is a proportional constant, E_g is the value of the band gap, n = 2 for a direct transition or 1/2 for an indirect transition, and [F(R_∞)hv]² = 0 is the Kubelka–Munk function which is defined as:²⁷

F(R_∞) = (1 − R)²/2R = K/S (2)

where R, K, and S are the reflection, the absorption, and the scattering coefficient, respectively. From the linear extrapolation in the inset of Fig. 5, the E_g value was estimated to be about 5.79 eV.

Fig. 6(a) depicts photoluminescence excitation (PLE) spectra of the as-prepared SYNPF:0.03Eu²⁺ and the photo-luminescence (PL) spectra of a series of samples SYNPF:xEu²⁺ (x = 0.005, 0.01, 0.02, 0.03, 0.04, 0.05). By monitoring the emission 467 nm, it can be seen that the excitation spectra both show a broad band covering the region from 200 to 450 nm, which is assigned to the parity allowed 4f⁷(⁸S_7/2) → 4f⁶5d transition of Eu²⁺ ions. The excitation spectrum indicates that the phosphor matches well with the commercial NUV LED chips (360–410 nm).²⁸ Upon excitation at 293 nm, the emission spectrum shows a broad band ranging from 400 to 550 nm with the maximum at 467 nm due to the 4f⁶5d → 4f⁷(⁸S_7/2) transition of the Eu²⁺ ions. In addition, we can also see that the intensities of emission with different doping contents from the inset of Fig. 6(a). The emission of the Eu²⁺ increases gradually and reaches a maximum at x = 0.03. With further increment of Eu²⁺ concentration, the emission intensity begins to decrease due to concentration quenching. In general, the concentration quenching is mainly caused by non-radiative energy transfer processes (cross-relaxation) among Eu²⁺ ions, which occurs as a result of an exchange interaction or a multipole–multipole interaction.²⁹ In order to identify the type of interaction mechanism, it is necessary to obtain the critical distance (R_c) that is the critical separation between the donor (activator) and acceptor (quenching site). According to the theory of Blasse, the critical transfer distance (R_c) of energy transfer can be calculated by the critical concentration of the activator ion:³⁰

where V is the volume of one unit cell, N is the number of the cationic sites occupied by activators in one unit cell, and x_c is the critical concentration of the activator ion. For SYNPF host, V = 564.795 ų, N = 6 and x_c = 0.03. Therefore, by using eqn (3), the R_c value of Eu²⁺ can be quickly obtained to be 18.17 Å. According to theory of Van Uitert, if the critical distance between the sensitizer and activator is shorter than 5 Å, the energy transfers type is exchange interaction.³¹ The above calculation result suggests that the energy transfer among Eu²⁺ ions in SYNPF:Eu²⁺ phosphor does not occur in this case. According to Van Uitert, if energy transfer occurs among the same type of activators, the intensity of the multipolar interaction can be determined from the change in the emission intensity from the emitting level that has the multipolar interaction. The emission intensity (I) per activator ion can be expressed as:³²where I is the emission intensity, x is the concentration of the activator ions above the concentration quenching point, β and K are constants for the same conditions, and Q is the function of multipole–multipole interaction for 6 (dipole–dipole), 8 (dipole–quadrupole) or 10 (quadrupole–quadrupole). As shown in Fig. 6(b), to obtain a correct Q the dependence of log(I/x) on log(x) is plotted, and it yields a straight line with a slope of −Q/3. The fitting result for Eu²⁺ emission centers, which is corresponding to the high Eu²⁺ concentration SYNPF:xEu²⁺ phosphor compositions, is shown in Fig. 6(b). Based on the fitting results in Fig. 6(b), the slope is −1.7, and the value of Q can be calculated as 5.1, which is approximately equal to 6. This indicates that the dipole–dipole interaction is the major mechanism for concentration quenching in SYNPF:Eu²⁺ phosphor.

Fig. 6 (a) PLE spectra of SYNPF:0.03Eu²⁺ and PL spectra of SYNPF:xEu²⁺ (x = 0.005, 0.01, 0.02, 0.03, 0.04, 0.05); the inset shows the variation of emission intensity as a function of doped Eu²⁺ molar concentration. (b) The relationship of lg(x) versus lg(I/x) for SYNPF:xEu²⁺ (x = 0.03, 0.04, 0.05) phosphor.

The excitation and emission spectra of SYNPF:0.03Eu²⁺ and SYNPF:0.12Mn²⁺ samples are shown in Fig. 7(a) and (b), respectively. The excitation spectrum of SYNPF:0.12Mn²⁺ includes several bands from 250 to 500 nm peaking at 341, 361, 405 and 462 nm, which originated from the ground state ⁶A₁(⁶S) to the excited states ⁴E(⁴D), ⁴T₂(⁴D), [⁴A₁(⁴G), ⁴E(⁴G)], and ⁴T₁(⁴G), respectively. Upon 405 nm excitation, the SYNPF:0.12Mn²⁺ phosphor shows an emission band ranging from 500 to 700 nm due to the spin-forbidden ⁴T₁(⁴G)–⁶A₁(⁶S) transition of Mn²⁺ ions.³³ The overlap of Eu²⁺ emission and Mn²⁺ excitation spectra can be readily observed from the color area in Fig. 7(a), which demonstrates the possibility of resonance type energy transfer from Eu²⁺ to Mn²⁺ ions in this host. The PLE and PL spectra of the SYNPF:0.03Eu²⁺, 0.12Mn²⁺ phosphor are shown in Fig. 7(c). At the irradiation of 293 nm, the PL spectrum exhibits both the Eu²⁺ and the typical Mn²⁺ emissions. The phosphor shows absorption of both the Eu²⁺ and Mn²⁺ ions, monitored at 467 nm which is the typical emission of the Eu²⁺ ions. To avoid the existence of the Eu²⁺ emission at 467 nm, we chose another emission peak at 567 nm as monitoring wavelength which is beyond the emission range of the Eu²⁺ ions. From Fig. 7(c), we can see that the PLE spectrum is similar to that monitored at 467 nm. The above analysis on the PLE and PL spectra of the SYNPF:0.03Eu²⁺, 0.12Mn²⁺ phosphor proves the occurrence of the energy transfer from the Eu²⁺ to Mn²⁺ ions.

Fig. 7 PLE and PL spectra of (a) SYNPF:0.03Eu²⁺, (b) SYNPF:0.12Mn²⁺, and (c) SYNPF:0.03Eu²⁺, 0.12Mn²⁺ phosphors.

3.3 Luminescent properties and energy transfer of Eu²⁺/Mn²⁺ co-doped SYNPF phosphors

In order to further understand the energy transfer between the Eu²⁺ and Mn²⁺ ions, the fluorescence decay curves of Eu²⁺ with different Mn²⁺ doping contents (0, 0.03, 0.06, 0.09, 0.12, 0.15), which are monitored at 467 nm and excited at 370 nm, are illustrated in Fig. 8(a). The entire decay curves can be fitted to a typical second-order exponential decay method through the following equation:³⁴

I(t) = A₁ exp(−t/t₁) + A₂ exp(−t/t₂) (5)

where I is the luminescence intensity at time t, A₁ and A₂ are constants, t is the time, and t₁ and t₂ are the decay time for rapid and slow lifetimes for exponential components. Then, the average decay lifetimes can be calculated as follows:

Fig. 8 (a) Decay curves of Eu²⁺ emission monitored at 467 nm for SYNPF:0.03Eu²⁺, yMn²⁺ (y = 0, 0.03, 0.06, 0.09, 0.12, 0.15) under excitation at 370 nm. (b) The fluorescence lifetime of Eu²⁺ ions and energy transfer efficiencies from Eu²⁺ to Mn²⁺ as a function of Mn²⁺ concentration.

Based on the above eqn (6), the average decay times (τ) are calculated to be 25.87, 24.03, 21.42, 19.86, 15.39 and 9.45 ns for SYNPF:0.03Eu²⁺, yMn²⁺ with y = 0, 0.03, 0.06, 0.09, 0.12 and 0.15, respectively. It can be seen that the decay lifetime of the Eu²⁺ ions decreases monotonically with an increase in Mn²⁺ doping concentrations, which strongly demonstrates the energy transfer from Eu²⁺ to Mn²⁺ ions.

Assuming that all the excited Mn²⁺ ions decay radioactively, the energy transfer efficiency (η_T) from Eu²⁺ to Mn²⁺ can be obtained according to the data of the average lifetimes by the following equation:³⁵

η_T = 1 − τ/τ₀ (7)

where τ and τ₀ are the decay times of the sensitizer Eu²⁺ in the presence and absence of the activator Mn²⁺, respectively. As shown in Fig. 8(b), the energy transfer efficiencies increase gradually with increasing Mn²⁺ concentration. The value of η_T reaches the maximum of 63% for the emission center monitored at 467 nm when y = 0.15, indicating that the energy transfer from the Eu²⁺ to Mn²⁺ is efficient.

In an effort to understand the energy transfer process, Fig. 9 shows the corresponding energy level scheme of SYNPF:Eu²⁺, Mn²⁺ with the optical transitions. When the Eu²⁺ ion is irradiated by NUV light, the excited electrons shift to the excited state (4f⁶5d) of Eu²⁺ and nonradiatively (NR) relaxes to the lowest 5d crystal field splitting state, and then the electrons of the Eu²⁺ ions shift from the excited state (4f⁶5d) to the ground state (4f⁷) with a 467 nm emission. As the similar values of the energy levels, an energy transfer is expected to take place from the excited 5d state of Eu²⁺ to ⁴A₁, ⁴E(⁴G) level of Mn²⁺ in the SYNPF host. Mn²⁺ ion receives the energy transferred from the excited Eu²⁺, the electrons in the Mn²⁺ ion from the ground state are excited into ⁴A₁, ⁴E₁(⁴G) energy levels. Then, the excited free electron relaxes to the excited state ⁴T₁(⁴G) through ⁴T₂(⁴G) intermediate energy level in a nonradiative process, followed by a radiative transition from the excited state of ⁴T₁(⁴G) to the ground state of ⁶A₁(⁶S), with a typical emission of Mn²⁺ located at 567 nm.³⁶

Fig. 9 The schematic of energy transfer in SYNPF:Eu²⁺, Mn²⁺.

To provide further evidence of the phenomenon that the energy transfer from the Eu²⁺ to the Mn²⁺ ions occurs in the SYNPF host, the emission spectra of SYNPF:0.03Eu²⁺, yMn²⁺ (0 ≤ y ≤ 0.15) excited at 365 nm are presented in Fig. 10. The intensity of the Eu²⁺ emission (467 nm) decreases with increasing doped Mn²⁺ content. In contrast, the PL spectra of the Mn²⁺ emission (567 nm) increases until the maximum is reached at y = 0.12, and then the emission intensity decreases (inset, Fig. 10). The decrease in emission intensity beyond a critical concentration of active ions can be explained by concentration quenching. This is mainly caused by energy transfer among the active ions in the SYNPF lattice, which results in the nonradiative energy transfer among Mn²⁺.³⁷

Fig. 10 PL spectra of SYNPF:0.03Eu²⁺, yMn²⁺ (y = 0, 0.03, 0.06, 0.09, 0.12, 0.15) phosphors under 365 nm excitation; the inset shows relative emission intensity of Eu²⁺ and Mn²⁺ in SYNPF:0.03Eu²⁺, yMn²⁺ phosphors.

In general, the energy transfer mechanism from the Eu²⁺ to Mn²⁺ ions via electric multipolar interaction and the relation can be adopted by the Dexter's energy transfer formula:³⁸

(I_s0/I_s) ∝ C^n/3 (8)

where I_s0 and I_s are emission intensity of Eu²⁺ in the absence and presence of Mn²⁺, respectively. C is the total doping concentration of the Eu²⁺ and Mn²⁺ ions. I_s0/I_s ∝ C^n/3, with n = 6, 8, and 10 corresponds to dipole–dipole, dipole–quadrupole and quadrupole–quadrupole interactions, respectively. The relationships between (I_s0/I_s) and C^n/3 are illustrated in Fig. 11, showing that the linear dependence of the dipole–dipole interaction is the best among the fitting results. This clearly indicates that the energy transfer from Eu²⁺ to Mn²⁺ follows a non-radiative dipole–dipole interaction. This result also proves the conclusion obtained by eqn (5) using the critical distance (R_c) of energy transfer.

Fig. 11 Dependence of I_s0/I_s of Eu²⁺ on (a) C^6/3, (b) C^8/3, and (c) C^10/3.

3.4 CIE coordinates, quantum efficiency and thermal properties of SYNPF:Eu²⁺, Mn²⁺

Fig. 12 and Table 2 show the Commission Internationale de L'Eclairage (CIE) chromaticity coordinates of the SYNPF:0.03Eu²⁺, yMn²⁺ phosphors, which were determined from the corresponding PL spectra at the excitation of 365 nm. As the content of Mn²⁺ increases from 0 to 0.15, one can see that the color tone of the obtained phosphors can be easily modulated from blue to white due to the emission intensity change between the 4f–5d transition of the Eu²⁺ ions and the ⁴T₁–⁶A₁ transition of the Mn²⁺ ions through the partial energy transfer. Accordingly, the CIE coordinates vary from (0.133, 0.148) to (0.352, 0.374) due to different emission components of the Mn²⁺ ions. Moreover, CCT as another important parameter for a phosphor is calculated using the following approximate formula reported by McCamy:³⁹ T = −437n³ + 3601n² − 6861n + 5514.31, where n = (x − 0.3320)/(y − 0.1858). From the results listed in Table 2, one can see that when the value of y increases to 0.12, a white light can be obtained with good CIE coordinates of (0.327, 0.351) and the CCT of 5738 K, which is lower than that of the WLEDs fabricated by the InGaN chips and YAG phosphors (CCT ≈ 7750 K).⁴⁰ These results indicate that the SYNPF:0.03Eu²⁺, 0.12Mn²⁺ phosphor could potentially be used as a white emitting source to meet the needs of illumination applications.

Fig. 12 CIE chromaticity coordinates and the luminescence photographs of SYNPF:0.03Eu²⁺, yMn²⁺ (y = 0, 0.03, 0.06, 0.09, 0.12 and 0.15) under excitation at 365 nm.

Table 2 Detailed CIE, CCT and QE data for SYNPF:0.03Eu²⁺, yMn²⁺ (y = 0, 0.03, 0.06, 0.09, 0.12, 0.15) phosphors excited at 365 nm

Sample no.	Mn^2+ content	CIE coordinates (x, y)	CCT (K)	QE
1	0	(0.133, 0.148)	382 987	37.2%
2	0.03	(0.194, 0.212)	833 202	30.2%
3	0.06	(0.241, 0.265)	20 082	27.8%
4	0.09	(0.286, 0.311)	8542	26.5%
5	0.12	(0.327, 0.351)	5738	24.4%
6	0.15	(0.352, 0.374)	4826	18.3%

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In general, for the practical application of phosphors for LEDs, the quantum efficiency (QE) of a phosphor is an important factor to be considered. The QE of SYNPF:0.03Eu²⁺, yMn²⁺ samples have been measured using an integrating sphere attached to the FL3-21 spectrometer. The QE values can be calculated by the following equation:⁴¹

where L_S is the luminescence emission spectrum of the sample, E_S is the spectrum of the light used for exciting the sample, and E_R is the spectrum of the excitation light without the sample in the sphere. Fig. 13 shows that the QE of the SYNPF:0.03Eu²⁺ phosphor estimated under 365 nm excitation; the results are given in Table 2. The QE values of SYNPF:0.03Eu²⁺, yMn²⁺ decrease with increasing contents of Mn²⁺, respectively.

Fig. 13 Excitation line of BaSO₄ and emission spectrum of SYNPF:0.03Eu²⁺ phosphor collected by using an integrating sphere.

As is well known, the phosphor converted LEDs generally work at a high temperature at about 150 °C. Thus, the thermal quenching is an important technological parameter for phosphors. Temperature-dependent relative emission intensity under 365 nm excitation of SYNPF:0.03Eu²⁺ and SYNPF:0.03Eu²⁺, 0.12Mn²⁺ is indicated in Fig. 14(a) and (b). The inset of Fig. 14(a) presented the detailed tendency to decrease for the emission intensities of Eu²⁺ ions in the SYNPF:0.03Eu²⁺ phosphor under different temperatures and as the temperature increased from room temperature to 150 °C, the emission intensity declines to 65% of the initial emission intensity. As shown in Fig. 14(c), the normalized emission intensity of the Eu²⁺ and Mn²⁺ ions in the SYNPF:0.03Eu²⁺, 0.12Mn²⁺ phosphor decreased to 82% and 73% of the initial value with increasing temperature up to 150 °C, respectively. By compared, the thermal stability of SYNPF:0.03Eu²⁺, 0.12Mn²⁺ gets better compared to that of Eu²⁺ single-doped phosphor.

Fig. 14 (a) SYNPF:0.03Eu²⁺ and (b) SYNPF:0.03Eu²⁺, 0.12Mn²⁺ PL spectra at different temperatures under 365 nm excitation. (c) The plots of ln[(I₀/I_T) − 1] versus 1/KT and (d) the thermal activation energy for SYNPF:0.03Eu²⁺, 0.12Mn²⁺ sample.

Generally, the decrease of emission intensity is ascribed to the thermal quenching of emission intensity via phonon interaction, in which the excited luminescence center is thermally activated through the crossing point between the ground and the excited states.⁴² To further investigate the thermal quenching phenomenon and obtain the activation energy for thermal quenching, the activation energy from the thermal quenching can be calculated using the Arrhenius equation:⁴³

where I₀ is the initial intensity, I(T) is the intensity at a given temperature T, ΔE is the activation energy for thermal quenching, c is a constant for a certain host, and K is the Boltzmann constant (8.629 × 10⁻⁵ eV). According to the equation, the activation energy ΔE could be calculated from a plotting of ln[(I₀/I_T) − 1] against 1/KT, where a straight slope equals ΔE. As shown in Fig. 14(d), ΔE was found to be 0.182 and 0.239 eV for Eu²⁺ and Mn²⁺, which are higher than that of Y₃Al₅O₁₂:Ce³⁺ phosphor (0.09 eV).⁴⁴ The relatively high activation energy ΔE results in a good thermal stability for this phosphor.

4. Conclusions

In summary, a series of Eu²⁺ single doped phosphors SYNPF:Eu²⁺ and Eu²⁺/Mn²⁺ co-doped phosphors SYNPF:Eu²⁺, Mn²⁺ were prepared through conventional solid state reactions. The energy transfer process of Eu²⁺ → Mn²⁺ has been investigated by the photoluminescence emission and excitation spectra, the decay curves, and the effect of the ratio of Eu²⁺ to Mn²⁺. The energy transfer mechanism was confirmed to occur via a dipole–dipole mechanism by the Dexter theoretical model. A white light could be generated by NUV LED pumping by fabricating a NUV 365 nm chip to pump a single-phase white-light SYNPF:0.03Eu²⁺, 0.12Mn²⁺ phosphor, producing a white light with a correlated color temperature of 5738 K and color coordinates of (0.327, 0.351). These results indicate that SYNPF:Eu²⁺, Mn²⁺ is a promising single-composition phosphor for application involving white-light NUV LEDs.

Acknowledgements

The present work was supported by the National Natural Science Foundations of China (Grant No. 21576002) and the Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions (Grant No. 201404030).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20699a

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